Magnetic resonance tomography (MR or MRT) is an imaging method for displaying tissue in the human or animal body and is based on the principle of nuclear magnetic resonance. Atomic cores in the examination chamber can be stimulated in an applied external magnetic field with electromagnetic radiation in the high-frequency range and emit this radiation shortly afterwards. This HF radiation is detected using a local coil, which can also be used to generate the stimulating radiation.
The local coils are disposed as close to the body of the patient as possible, to increase the signal to noise ratio (SNR) of the high-frequency measurement during MR acquisition. Generally the local coils can be designed specifically for different body regions. There are therefore body coils, head coils, spine coils and others. These specific local coils are tailored as closely as possible to the anatomical shape of the body region examined in each instance.
A further medical imaging method is positron emission tomography (PET). As a nuclear medicine method PET is particularly suitable for displaying biochemical processes in the body. A radionuclide is administered to the patient, which is then distributed in the body, thereby emitting radioactive radiation in the form of positrons. The two gamma-quanta emitted in the opposite direction as the positrons decay are detected by detectors. The detectors are generally disposed in an annular manner around the body.
Since the local coils for detecting the high-frequency signals are disposed close to the body during MR acquisition, with a combination of magnetic resonance measurements (MR) and positron emission tomography (PET) they may be in the beam path of the positron annihilation radiation from the PET acquisition and may therefore disrupt the sensitivity of its detection.
Local coils for MR measurements in particular in a combined MR/PET system are known for example in the form of the field generation unit in DE 10 2006 046 287 A1. With this field generation unit the HF antenna arrangement has a first part, which is integrated in a fixed manner in the examination tunnel, so that it is disposed below the inserted patient support, and a second part, which can be positioned on the patient support and can be moved in and out of the examination tunnel with this, the second part being embodied as rigid in form and having a hollow cross section, which is tailored to the object to be examined.
With this prior art the second part comprises local coils, which are disposed above the body or head of the patient and are therefore located between the body and a PET detector facility. However in some circumstances these local coils therefore obstruct the radiation generated by positron annihilation with PET. In other words the signals for PET imaging are partially attenuated by the local coils for MR acquisition and the PET result is therefore falsified. Unfavorable beam penetration through components, for example with vertical plastic housing walls, can even result in complete extinction of the PET radiation.
Attempts are made in different ways with the prior art to correct the partial attenuation of the PET beams in the PET measurement result.
With older PET devices the attenuation of PET radiation by the components in the examination chamber is measured by way of a rotating transmission source.
With a combination of PET and computed tomography (CT) the attenuation of the PET beams is measured directly with the CT device and stored in a so-called attenuation map (μ map). To correct the PET radiation in respect of attenuation by components in the examination chamber, this attenuation map (μ map) is used when reconstructing the PET images. No high-frequency antennas are used in the examination chamber with such devices. However such high-frequency antennas are required for the PET/MR combination, so in the MR and PET combination a transmission measurement involves additional outlay. The attenuation map is therefore set out and stored in a dedicated device. The attenuation map is then used as a function of measurement position in the correction of the measurement result.